The present invention relates to model engine glow plug filament temperature control, and in particular to a method and apparatus for measuring the temperature of the filament to be controlled and generating a control signal therefrom.
Small gas model engines, such as those found in radio-controlled airplanes, cars, and boats, have their fuel-air mixture ignited by a glow plug which screws into the head of the engine. When the engine is running in its normal range of RPM, the heat of combustion of the engine is sufficient to keep the tip of the glow plug and its filament red hot and thus to ensure ignition of the mixture when it is compressed in the cylinder.
However, when the engine is initially started, the glow plug is cold and must be heated in some way before it can ignite the fuel. This is the purpose of the glow plug filament. It is a coil of resistive wire in the tip of the glow plug, and the application of electric power to external points on the glow plug will cause the filament to glow red hot. Typically, 3 amperes is required. This electric power is applied only long enough for the engine to be started, because the heat of combustion will now keep the plug red hot and it is undesirable to have the plug heated by both means since the filament can be stressed and its life considerably shortened.
The engine can be started by manually flipping the propeller around (in the case of a model airplane) or, as is often done, a hand-held electric starter can be used to more rapidly and continuously flip the propeller around until the engine fires.
For many years the means most often used to light the glow plug was a 1.5 volt dry cell. This had numerous disadvantages, the foremost being loss of capacity in cold weather and short life due to the large current required.
Later advances in the art utilized instead a switching-circuit approach wherein a much higher voltage (usually 12 volts) is repetitively switched or pulsed into the glow plug, the duty cycle of this pulse train being initially adjusted to provide the correct amount of average power to the glow plug to keep it red hot. The advantage of the switching action is that much less average current is drawn from the battery, typically one-half ampere. Also, being either "on" or "off," the switching element does not dissipate much power and runs fairly cool.
The most recent advances in the art have in addition used temperature feedback directly from the plug filament to modulate the duty cycle of the pulse train. Since the filament's resistance decreases when its temperature decreases, the resistance can be sensed and any deviation from the desired value can be used to cause a correcting change in the pulse train duty cycle. This technique gives very easy starting of model engines under the most difficult circumstances, such as flooding, because the plug filament receives as much power as is needed to keep it red hot.
Problems still existing in the prior art circuits include the following. Stability with respect to ambient temperature and component aging is poor, requiring frequent readjustment of the temperature adjustment control. The operator of these prior art circuits often lacks confidence that the plug is indeed at the correct brightness, and he must remove it from the engine to examine it.
Another problem as yet unsolved in the prior art is the sensitivity of these complex circuits to drops in the main supply voltage. The battery used to power the glow plug heating device is very often the same 12 volt battery used to power the electric starter. The starter can draw 15 amperes when attempting to turn an engine. The 12 volt battery used often cannot sustain this without a substantial drop in terminal voltage. Drops to 8 volts are not uncommon. This occurs when it is most important that the glow plug filament be red hot. The prior art circuits have no provision for internal voltage stabilizing so that supply voltage drops can be dependably rejected. At low supply voltages their action becomes unpredictable.
An additional problem remaining in the prior art is that of turning off the pulses once the engine has started. To least affect the life expectancy of the filament it is necessary to remove as completely as possible all electric power from the filament immediately after the engine starts. The resistance-measuring technique will always require some small measuring current to flow through the filament, but the high-amperage pulses should be terminated. Also, in applications such as larger "scale" aircraft where the glow plug heating circuit may remain on board and permanently connected to the glow plug, it is desirable to minimize the current drawn from the battery, which also must be carried. This requires termination of the pulses.
However, it is not sufficient to merely terminate the pulses when filament resistance rises. In applications such as idling the engine, which may occur in larger-scale aircraft, or when "breaking-in" an engine by running it with an excessively rich fuel mixture, the filament-heating circuit is most useful if it can instantaneously turn off and on, for only one pulse if necessary, to assist the engine in keeping the glow plug red hot. The problems involved in a design of this type have not been resolved by the prior art. A totally automatic filament heating circuit that can turn the pulses off and on as required, without requiring the use of a throttle-position sensing micro switch, as has been done, is not available in the prior art.